Musical instrument amplifiers function to not only increase the power of an input signal presented to the amplifier in order to drive a speaker or other load but also to modify the signal through the introduction of distortion and compression. The usual method of introducing distortion and compression is to configure the amplifier to produce so much gain that one or more of the amplification stages within the amplifier are driven beyond the limits of linear operation. Simply making an amplifier with exceeding amounts of gain, however, ignores the intricate dynamics that occur when an amplifier is driven to and beyond the limit. In particular, the power supply's interaction with the amplifier's operation has a substantial impact on the sound coloration produced by the amplifier.
Audio amplifiers, for both musical instruments and sound reproduction, have power supplies that usually include a transformer having its primary winding connected to AC line voltage supplied by a power company. The transformer's secondary usually consists of three windings. The first winding is used to produce the high positive DC voltage, and it is normally considered the amplifier's main power supply. The second winding, which may either be a completely separate winding or a section of the first winding, is used to create a negative DC bias voltage for the power tubes in the amplifier. The third winding is used to supply power to the tube heaters. The present invention is concerned with the amplifier's main power supply which is referred to herein as the "power supply."
Manufacturers of musical instrument amplifiers have begun to recognize the effect the power supply has on the sound produced by the amplifier. The compression and distortion that is desirable in certain musical instrument amplifiers is caused by a "sag" in the power supply. The sound produced by amplifiers that exhibit large amounts of power supply sag is often described as "warm", "airy" and "forgiving". On the other hand, amplifiers having power supplies that do not sag very much produce sound described as "hard", "edgy" and "harsh".
All power supply circuits exhibit some amount of dynamic impedance that causes power supply sag. When the current drawn from a power supply increases, the dynamic power supply impedance causes the average level of the voltage produced by the power supply to decrease, or sag. This decrease in voltage is due to the increase in current drawn from the supply that causes an IR drop across the power supply impedance. The amount that the power supply's DC level sags for a given amount of current drawn from the supply is dependent on the power supply impedance. This impedance is dependent on the resistance of the transformer's primary and secondary windings, the impedance of rectifiers, and the values of smoothing circuit components.
The time behavior of power supply sag is also dependent on the power supply impedance. The notable aspects of the time behavior of power supply sag are 1) the rate at which the output of the power supply decreases when the current drawn from the power supply increases, i.e. sag rate, and 2) the rate at which the output of the power supply increases back up to its DC level when the current drawn from the power supply decreases, i.e. recovery rate. Vacuum tube rectifiers exhibit a relatively large dynamic impedance when compared to solid state rectifiers. Replacing modern solid state rectifiers with old style vacuum tube rectifiers has become commonplace when the compressive sound of 1950's and 1960's style amplifiers is desired. The large vacuum tube rectifier impedance causes the decrease in power supply voltage to become quite pronounced when a large amount of current is drawn from the supply. The average level of the power supply voltage can decrease by 20% or more in some cases. This large decrease limits the maximum amount of power an amplifier can deliver, thereby compressing the output signal and causing distortion in the amplifier's output.
The disadvantages of vacuum tube rectifiers include size, expense, frequent need of replacement, and the need for separate high current filament supplies that dissipate high amounts of heat. Additionally, the current capacity of most available vacuum tube rectifiers is so limited that some high power amplifiers require two or more of these rectifiers. To avoid these problems with vacuum tube rectifiers, some manufacturers include a large resistor in series with the power supply output to increase the power supply's dynamic impedance while still utilizing solid state rectifiers. Another method of increasing the power supply's dynamic impedance while using solid state rectifiers has been to wind the secondary of the power supply transformer with very high resistance wire, effectively placing a large resistor in series with the power supply output.
While increasing the dynamic impedance of a solid state rectified power supply enables an amplifier to produce sonic characteristics similar to that of older amplifiers, high power supply impedance greatly reduces the maximum amount of power that an amplifier can deliver to a load, (especially at the low end of the audio spectrum). To avoid this problem, some manufacturers include two sets of power supply rectifiers, vacuum tube and solid state. The musician can then switch between these two sets of rectifiers to make the amplifier sound two different ways. In power supplies utilizing solid state rectifiers with a fixed resistor in the power supply, some amplifiers allow the resistor to be switched in and out of the circuit. Both the multiple rectifier and the fixed power supply resistor approaches only allow the musician to choose between two amounts of power supply sag for a given amount of current drawn from the power supply. That is, the sag magnitude is not completely adjustable. Using a very high power variable resistor in conjunction with solid state rectifiers would increase the power supply circuit's flexibility, but at very high cost, as well as increased weight and space requirements. Consequently, this solution is not practical. An interesting application of power supply sag is shown in U.S. Pat. No. 4,713,624 issued Dec. 15, 1987 to Smith. The main idea behind this patent is to increase the effective power supply resistance of the screen grid power supply in an amplifier using pentode power tubes in a push-pull configuration. The plate current in a pentode is at least 10 times more dependent on the screen voltage than on the plate voltage. Therefore, if the screen voltage decreases substantially due to power supply sag, the maximum plate current, and therefore load current, that the amplifier can produce is also decreased. This decrease in maximum load current compresses the output signal and introduces distortion in the amplifier's output. Additionally, increased screen power supply impedance also decreases the sag rate and decreases the recovery rate. These rates determine the time behavior of the power supply sag and are very critical in the overall sound produced by an amplifier. While U.S. Pat. No. 4,713,624 recognizes the important role that power supply impedance plays in the overall behavior and sound of an amplifier, the patent does not allow for the amount of power supply sag to be adjustable. Additionally, the power supply sag rate and recovery rate in the Smith patent, or for that matter any other power supply configuration, are not adjustable.
In many sound reproduction amplifiers (e.g. home stereos) the power supplies are regulated. Regulated power supplies offer many benefits over unregulated power supplies. The first benefit of a regulated power supply is that power supply ripple is reduced. Power supply ripple can induce unwanted hum in the amplifier's output. The second advantage of regulated power supplies is that the voltage produced by the supply is independent of the voltage delivered by the power company (within limits). The operating point of the various devices within an amplifier is directly dependent on the DC level produced by the amplifier's power supply. If these operating points change, the amplifier's sound can change. Having a consistent power supply is very important in musical instrument amplifiers since repeatable amplifier behavior is desired by musicians as they travel to various locations for live performances.
The third advantage of regulated power supplies in most applications is that the dynamic impedance of these supplies is basically zero. That is, the voltage produced by the supply is independent of the current delivered by the power supply, i.e. no sag. This last property of regulated power supplies is a disadvantage when the power supply is used with a musical instrument amplifier. Therefore regulated power supplies are generally not used with musical instrument amplifiers, as a matter of fact, many home stereo amplifiers, especially tube types, do not use regulated power supplies because the manufacturer feels that unregulated supplies add a "warmth" to the amplifier's sound.
An important modification of unregulated power supplies used in musical instruments has come from some enterprising musicians. These musicians have found that by varying the AC power delivered to an amplifier, they can dramatically change the amplifier's sound. This change in AC power is accomplished by plugging the amplifier into a variable transformer, which is then plugged into the wall outlet. By altering the AC voltage delivered to an amplifier, the basic sound and maximum signal level produced by the amplifier is changed. This alteration has been embraced in an amplifier design in U.S. Pat. No. 5,091,700 issued Feb. 25, 1992 to Smith where multiple taps on the primary of the power transformer are provided in order to produce various levels of power supply DC output voltage.
The drawback of both the variable transformer as well as the multiple tapped primary concepts is that the voltages present on the other secondary windings are being modified. Altering the negative bias voltage of the output tubes can lead to crossover distortion, or dramatically increased DC tube current. Crossover distortion is very unpleasant and not desired, while increased DC tube current can lead to premature tube failure.
To eliminate the problem of altering the negative bias and heater supplies with a multiple tapped transformer primary, these two voltages could be produced from a second transformer that does not have a multiple tapped primary. However, the use of a second transformer would dramatically increase the cost and space requirements of these supplies. Additionally, the multiple tapped primary does not allow the DC voltage produced by a power supply to be continuously variable, and adding taps to a transformer increases the cost of the transformer.
In summary, all known power supply circuits for musical instrument and sound reproduction amplifiers suffer from one or more of the following disadvantages:
(a) The amount of power supply sag due to a given amount of current drawn from the supply is fixed at a single or at most two values. PA1 (b) The time behavior of the power supply sag is not adjustable by the musician. PA1 (c) The amount of power supply sag and time behavior of the power supply sag are interrelated. PA1 (d) In unregulated power supplies, the average voltage produced by the supply is dependent on the voltage delivered by the power company. PA1 (e) In unregulated power supplies, hum is present in the power supply output which can produce hum in the amplifier output. PA1 (f) In unregulated power supplies, the DC level of the voltage produced by the supply is not adjustable without modifying the negative bias and heater supplies unless a second transformer is used. PA1 (g) In unregulated power supplies, the DC level of the voltage produced by the supply is not continuously adjustable unless a variable transformer is used. PA1 (h) In regulated power supplies, the voltage produced by the supply does not decrease with increased supply current (i.e. the output voltage does not sag). PA1 (a) Independence of the power supply output voltage from fluctuations in the power company line voltage. PA1 (b) Elimination of ripple from the output of the power supply. PA1 (c) User controllable DC power supply voltage. PA1 (d) User controllable power supply sag magnitude. PA1 (e) User controllable rate of power supply sag. PA1 (f) User controllable rate of power supply recovery. PA1 (g) A decoupling of the DC level, power supply sag magnitude, power supply sag rate and power supply recovery rate controls.